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Shielding calculation and nuclide vector
2

Shielding calculation and nuclide vector

Shielding calculation and nuclide vector

(OP)
I have to make a shielding calculation of some radioactive scrap metal in a cask, using Geant4 (hence Monte-Carlo).
I will be given a nuclide vector and I'll have to calculate the ambient dose equivalent rate outside of the cask.
The nuclide vector I will be given will be something like:

Co60 X Bq
Cs137 Y Bq
etc...

The problem is that I've been told that I must not only consider the activity of the expressly listed nuclides but also of any eventual daughter in equilibrium with the parent, even if not expressly listed.
The question is:
how do professionals usually deal with this problem ? How do they calculate the daughters' activities ? By hand ?
In principle I can try to calculate by hand all the equilibria related to each listed nuclide, but there are nuclide having many different decay branches, each with its own decay constant: it's going to be laborious.
I've been asked to prepare a budget of working hours for the simulation. I'm afraid that such a preliminary work only to calculate all the possible equilibria is going to deeply affect the budget.
Consider that just a nuclide like Co60 can decay through beta-minus to two possible excited states of Ni60, each in turn with its own gamma-decay-constant...

Thank you in advance.

RE: Shielding calculation and nuclide vector

How long is the calculation to cover? (How many years?)

What is the history of the nuclide, and the power history of the reactor that created the nuclide?

RE: Shielding calculation and nuclide vector

(OP)
I don't have the nuclide vector yet, because now I'm not yet asked to make the shielding calculation, but only to forecast how many working hours I need to do it. On the basis of my forecast my employer would make an offer to the client.

The only information I dispose of now is that it's scratch metal from the core of an old BWR: mainly control rod guide thimbles and a dismembered (pulled apart) steam drier. Moreover, the client provided some values of the ambient dose equivalent rate around one of the control rod guides, just as example:

XXX mSv/h at the head
YYY mSv/h at the middle height
ZZZ mSv/h at the foot of the control rod guide

But no nuclide vector and no burn-up history. Somebody will have to infer a nuclide vector on the basis of the doses and the reactor's history, but I think it's not going to be my task.
My nightmare is that if they tell me that in the cask there are f.e. XXX Bq of 55Cs137, then I am expected to infer by myself how many Bq of 56Ba137m and of 56Ba137 there are, to input in Geant4, assuming secular equilibrium. And the same goes for each other nuclide of the vector.
I ask myself if all Monte-Carlo professionals do this preliminary work by hand when they have to do any shielding calculation of some scratch metal in a cask.

RE: Shielding calculation and nuclide vector

There are tables available that show standard decays based on the initial material. However, it appears that you're uncomfortable doing this task. Perhaps you should talk to your supervisor about some of your concerns on how to go about this. If he's putting in a bid to do the calculation, then he will want to know your comfort level with the estimate.

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RE: Shielding calculation and nuclide vector

I am a little surprised at your reluctance. These are "standard" calculations for nuclear engineers; There were 3 versions of the problems required for my PE exam.

I have not used the constants and the assumptions and the equations for a while. Clearly, your boss feels that "you" (or your company actually) does have the ability to learn the equations, make the spreadsheets with the various inputs, then present a report with the outputs.

So. If I did 3x similar problems in predicting heat rates based on power histories in two 4 four-hour sessions using a pen and calculator (because spreadsheets had not yet been invented when I took the test), how long will it take you - and you have not told us your abilities and your training! - to write a spreadsheet with those equations and the needed input variables?

Then add a margin for rework and the learning curve. To repeat the above: Have YOU talked to your boss yet? What did she say? Why did she select YOU to do this calc?

RE: Shielding calculation and nuclide vector

(OP)
racookpe1978, EnergyMix,
thank you very much for your help.

I have been given a sheet reporting an example of nuclide vector contained in a cask. But it's just an example.
There are the following nuclides: H-3, C-14, Mn-54, Fe-55, Co-60, Ni-63, Ag-110m, Sb-125, I-129, Cs-134, Cs-137, Pu-241, Ra-226, Th-232, U-235, U-238, Pu-238, Pu-239, Pu-240, Am-241, Cm-244.

Some of these nuclides have quite simple decay-schemes, where the daughter-nuclides have a higher decay constant λ than the mother: here you have secular equilibrium and the calculation is simple.
But some of them have quite complicated decay-schemes, where the conditions of secular equilibrium apply only in part and only to certain decay branches...I guess in those cases one should write the differential equations and solve them.
I can do it but I need time because I am not specifically trained. racookpe1978, I believe that you did that work in two 4-hour sessions, but I guess that you have been preparing weeks for that exam. In the last 3 months I have been learning Geant4.
More over, if I need say 5 work-days to do the Monte-Carlo simulation (and just because it's the first time that I use Geant4) can I reasonably ask for 2 additional days (+ 40% time) only to come to terms with the decay chains and the differential equations ? Is it normal ? I'm skeptical.

As you suggest, I have explained the problem to my boss yesterday and he told me that, effectively, he's not sure that we have to do this type of preliminary work: he's going to check and he will give me some feedback.

RE: Shielding calculation and nuclide vector

You are NOT paid for "exploratory" work, nor for "analyzing and making a bid". Those are overhead hours, charged to the company as a "cost for doing business". Sorry, that's the way the world works.

then, after you make the bid and the sales calls and the sales presentations and the ten weeks of scheduling and re-calls and additional presentations.... THEN you get to charge hours against the "project number." At that time, the company bookkeepers add a 'multiplier" to your salary for every direct charge hour you place against that job order.

If you get paid 50.00 per hour, the company needs to be charging the customer 200.00 to 350.00 per hour for the up-front money that effort costs. Plus the bookkeeper overhead rate, your bosses' overhead rates, and the VP's overhead's rate.

It is your responsibility to charge the hours worked against each job order accurately and honestly.

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